When acquiring digital video with a camera equipped with a lens, various problems may be caused by lens aberration. One such type of lens aberration is chromatic aberration, which is caused by the fact that the refractive index of light with respect to lens material varies due to its wavelength, which typically results in false color at an image capture surface, such as a digital image sensor of the camera. Lens aberration is the major phenomenon responsible for what is often referred to as color fringing. Often, the color fringing is most prominent in the blue/violet wavelength region. Thus, the term purple fringing is often used. Color fringing may, however, occur at any color. Color fringing appears as borders of false color around objects in an acquired digital image frame. The borders often appear around over-exposed areas near the edges of the acquired digital image frame. The problem may be exaggerated when performing digital image processing to enhance visibility of the scene and may therefore be a considerable problem for many applications where post-processing of digital image frames is needed.
Some of the color fringing problem come from longitudinal chromatic aberration in imaging optics of the camera, the longitudinal aberration being a result from blue light being refracted more than green or red light. Thus, blue light, for example, may be focused before the image plane, which results in the blue light being out of focus at the image plane. Some color fringing may also come from lateral chromatic aberration, which results from obliquely incident light being focused to different spatial positions within the image plane. Hence, all colors will be focused to the same plane, but the foci are not placed along the optical axis of the incident light.
There are ways to reduce color fringing due to color aberration, such as using high-quality lenses made from special material, such as fluorite. However, since high cost is required to manufacture such lenses, the price may be too high for many applications. Another way to reduce color fringing is to analyze and post-process the digital image frames. Several methodologies exist to minimize color fringing in existing digital image frames by means of post-processing. However, this is not a trivial problem as it is not readily apparent which regions in a digital image frame that suffer from the problem. One approach is to correct for color fringing at areas which are likely to suffer from it. One typical example where color fringing occurs is close to an over-exposed area in the periphery of a digital image frame. As color fringing often is purple, an approach may be to correct for purple fringing at borders of over-exposed areas of a digital image frame. This approach may work in some cases, but there are also cases where the approach may lead to inappropriate action. A reason is that not all pixels close to over-exposed areas are suffering from color fringing. One example of such a situation is a cumulus cloud on a blue sky. The cumulus cloud may appear to be an over-saturated area and, consequently, an image analysis may falsely interpret the blue sky at the border between the sky and the cloud as regions suffering from color fringing. There is thus a need in the art for an improved method for detecting and reducing the effects of color fringing in digital video.